The group is also supported by morphological characters, and can
be considered as including all animals that shed their exoskeleton (see ecdysis). Groups corresponding
roughly to the Ecdysozoa had been proposed previously by Perrier in
1897 and Seurat in 1920 based on morphology alone.

The group has been contested by a significant minority of
biologists. Some have argued for groupings based on more
traditional taxonomic techniques,[4]
whilst others have contested the interpretation of the molecular
data.[5][6]

Group
characters

A phylogenetic tree of the Ecdysozoa hypothesis as
suggested by Dunn et al. (2008)

The most notable characteristic shared by ecdysozoans is a
three-layered cuticle
composed of organic material, which is periodically molted as the
animal grows. This process of molting is called ecdysis and gives the group its name. The
Ecdysozoans lack locomotory cilia, produce mostly amoeboid sperm, and their embryos do not undergo spiral cleavage as in most other
protostomes. Various other features are found in the group, for
instance, tardigrades, pycnogonids and roundworms have a triradiate
pharynx.

The Ecdysozoa include the following phyla: Arthropoda, Onychophora, Tardigrada, Kinorhyncha, Priapulida, Loricifera, Nematoda and Nematomorpha. A few
other groups, such as the gastrotrichs, have been considered possible
members but lack the main characters of the group, and are now
placed elsewhere. The Arthropoda, Onychophora and Tardigrada have
been grouped together as the Panarthropoda because they are
distinguished by segmented body plans.[7] Dunn
et al. in 2008 suggested that the tardigrada could be
grouped along with the nematodes, leaving Onychophora as the sister
group to the arthropods.[3]

The non-panarthropod members of Ecdysozoa have been grouped as
Cycloneuralia
but they are more usually considered paraphyletic.

Criticism

The grouping proposed by Aguinaldo et al. is not
universally accepted. Some zoologists still hold to the original
view that Panarthropoda should be classified with
Annelida in a group called the Articulata, and that
Ecdysozoa are polyphyletic. The highly derived roundworms, with their many highly derived
parasitictaxa and a considerable number of autapomorphies continue to pose problems,
and are one of the most contested inclusions of grouping.

Cuticularepithelia are widely spread over diverse
phyla of invertebrates (including some groups outside Ecdysozoa,
such as annelids and molluscs, where it acts as
a skin instead of an exoskeleton) and show a considerable degree of
variation. They are believed to have evolved independently, at
least in some groups. In Nematoda and Panarthropoda, the cuticle is
different in both chemical composition and ultrastructure. While
the cuticle in arthropods like insects contains chitin, or can be a combination of both chitin
and keratin in crustaceans,
chitin has never been found in the complex cuticle of Nematoda
which is a fibrous and multilayered structure made of collagen and keratin of types
unique to the Nematoda.

Molecular evidence for the monophyly of Ecdysozoa is also
ambiguous.[5][8]

One of the proposed solutions is to regard Ecdysozoa as a
sister-group of Annelida,[9] however
the controversy is still far from closure.[10]